We are generally accustomed to not seeing the mezuzah itself, but its case. This is a tube of metal, wood, or plastic which contains the parchment scroll. Many do not fully realize just how much skill and labor goes into the writing of a mezuzah.
The only material that a mezuzah can be written on is parchment. The parchment used for mezuzahs or tefillin is not ordinary parchment.
From the very first stages, the parchment must be worked for the sake of the mitzvah. This means that the scribe must have in mind (and may verbally express) that he is preparing the parchment in order that a Torah scroll, tefillin, or mezuzah will be written on it.
The Torah says in connection with Tefillin (Exodus 13:9) “...In order that the Torah of G‑d should be in your mouth.” The Sages explain that this means that the parchment for the Tefillin scroll must be made from an animal permitted to be eaten according to Jewish law -- a kosher animal. However it is not required to be ritually slaughtered. Usually the skin of a calf or a lamb is used.
The skin is first soaked for a few hours in water and cleaned. Then the scribe places it in a barrel of lime where it remains between one and three weeks. By Jewish Law it must remain in the lime until most of the hair is easily removed.
The skin is then passed between the rollers of a depilatory machine which scrapes away the last traces of hair.
The skin is now stretched on a frame. The lime must be cleaned from the parchment otherwise it might harm the parchment or render it transparent, which would be unsuitable for the writing of a mezuzah or tefillin.
Drying the skin requires great care, in order that the parchment should have the delicate texture required. The skin is stretched taut on the frame and allowed to dry under a gentle indirect heat; shaded sunlight is the ideal. The tension of the skin during the drying process alters the arrangement of its fibers. This stage is crucial in the transition from animal hide to parchment.
The quality of parchment which has now been achieved is called g’vil. This is a heavy, thick substance. Jewish law enjoins us to use a more delicate quality of parchment called klaf. To understand the difference between g’vil and klaf, let us for a moment consider the structure of animal skin.
The thin outer layer is the epidermis, through which the hairs protrude. Beneath this is the dermis or corium. Its structure is a network of collagen fibers, and it contains the hair roots. The innermost layer which is called the “under-skin” consists of loose connective tissues in which are contained sweat and fat glands, blood vessels and muscular fibers.
The epidermis is unsuitable for parchment and is removed early in the process. In the case of sheepskin, two remaining layers of skin can, with great skill, be separated from each other. The dermis is then called klaf, and the under-skin is called duchsustus.
Another way to obtain the klaf is to scrape away the under-skin while the skin is still stretched on the frame.
A special knife is used for this consisting of a large semicircular blade set in a thick mount which is held with both hands. The setting of the blade at exactly the right angle is very important. A mistake at this stage might split the skin; after this it may well not be possible to continue to process it and all the scribe’s work will have been wasted.
The klaf is now taken off the frame and every square inch of it is rubbed carefully with different grades of pumice stone and chalk in order to obtain the rich smooth surface which is necessary for writing. After at least a few weeks maturing in a dry atmosphere the parchment is at last ready for the writing of a mezuzah or tefillin.
The main ingredients of the special ink used by the scribe are gall nuts, or the gallic acid derived from these nuts; gum arabic, a resin substance; and copper vitriol, a bluish stone which gives the ink the blackness required. The gall nuts and the resin are cooked in water for about an hour. Then the vitriol is added and the mixture is boiled until half remains.
Further details of this process are, however, a closely guarded secret. Through tradition passed to him by his teachers, and through knowledge gained over years of experience, each scribe has his own formula for making an ink with the desired qualities of clarity, easy flow, intense color and tackless film on drying.
The pen used for mezuzahs or tefillin is generally a quill from a kosher bird, a goose or a turkey. The point is carefully cut so that by turning the pen and varying the pressure, the scribe can write thick as well as very thin lines with one stroke.
The first step in the actual writing of the mezuzah is the ruling of twenty-two lines with a bone stylus.
The letters of the sacred text will be written suspended from these lines (rather than resting on the lines). The ruling of these lines is not just to guide the scribe’s hand, but an integral part of the scribal laws handed down from the time of Moses.
In Exodus 6:4 the Torah instructs, "U'ktavtam..." The literal translation is, "And you shall write them [on the doorposts of your house]." The Sages explain that this word can be read as two words: ktav tam -- a perfect script. The special script, in which the mezuzah, like the tefillin and Torah scroll, is written, has been very precisely defined by the Sages.
In the Code of Jewish Law, the exact form of each letter, from alef to taf, is clearly described. Each letter must be written perfectly in accordance with these laws, for the slightest flaw can render the whole mezuzah or tefillin not-kosher.
The letters may not touch each other, but should be separated by at least a hairsbreadth of space. The space between two words should be the size of a letter yud. If two words are written so closely together that a child learning to read thinks they are one word, the mezuzah or tefillin is not-kosher. This is also the case if a large space in the middle of a word makes it seem like two words.
In summation: There are many details and precepts in regards to creating and writing a mezuzah, tefillin or Torah scroll. We have only outlined here a few of the precepts, to give a feeling of what kind of handicraft goes into creating these holy objects.